The invention relates to the field of aircraft ground proximity warning systems and more particularly to systems providing warnings of excessive aircraft barometric descent rates.
In prior art systems as represented by Bateman U.S. Pat. No. 3,946,358; Bateman U.S. Pat. No. 3,947,808; and Bateman U.S. Pat. No. 4,060,793 each of which is assigned to the assignee of this application, the relationship between altitude above ground and aircraft descent rate is utilized to generate a warning when the aircraft descent rate is excessive for its altitude above the terrain. In U.S. Pat. No. 3,946,358 for example, an excessive descent rate warning system is disclosed that utilizes a linear relationship between aircraft altitude above terrain and aircraft descent rate wherein the greater the aircraft's altitude above the terrain, the greater the rate of descent that is required to activate the warning. In the excessive descent rate warning system disclosed in U.S. Pat. No. 3,947,808 at descent rates above a predetermined value, a warning is generated for a selected time of impact with the ground assuming the aircraft continues at that particular descent rate. U.S. Pat. No. 4,060,793 discloses a descent rate warning system wherein an advisory voice warning is provided for a first relationship between descent rate and altitude above terrain and a second type of imperative voice warning is generated for greater descent rates at lower altitudes where the danger of aircraft impact with the terrain is more significant.
Studies of the effectiveness of ground proximity warning systems in actual airline operations have indicated that pilot acceptance and confidence is a substantial factor in judging the value of the ground proximity warning systems from the safety standpoint. In judging the effectiveness of a ground proximity warning system one of the primary criteria is the number of nuisance warnings generated by the system. A nuisance warning is generally defined as a warning generated by the system when the aircraft is essentially operating in conformity with offically approved operating procedures such as FAA regulations and the aircraft is not in any immediate danger of impacting the terrain. One source of nuisance warnings in the excessive descent rate warning mode of ground proximity warning systems occasionally occurs during landing approaches where the terrain slopes downwardly towards the runway and it is necessary for the aircraft to maintain a fairly high descent rate in order to make a normal approach. Runway 27 at San Diego, Calif., is one example of such an approach wherein the terrain slopes downwardly towards the end of the runway and the aircraft must perform a relatively steep approach close to the terrain for periods of time that may in some cases extend as long as 30 seconds. These approaches will occasionally cause the aircraft to penetrate the warning envelopes disclosed in the above mentioned references and in particularly the sink rate warning envelope of U.S. Pat. No. 4,060,793. Warnings under these circumstances can be considered nuisance warnings since the aircraft is operating within officially approved flight procedures and is in no danger of impacting the terrain.
However, merely decreasing the sensitivity of the warning system by increasing the descent rate required to generate a warning or decreasing the altitude at which a warning is generated would tend to eliminate warnings under circumstances where historical analysis has shown that accidents have actually occurred thus decreasing the effectiveness of the system in warning of hazardous flight profiles. Therefore, in order to enhance pilot acceptance of the systems, it is considered desirable to eliminate this type of nuisance warning generated in areas where the aircraft is traversing terrain that is sloping downwardly toward the runways while at the same time not decreasing the effectiveness of the system in warning of dangerous flight conditions.